One method of making rare earth-iron based magnets is by jet casting molten alloy through a small orifice onto a rapidly moving quench surface. This is disclosed, for example, in U.S. Pat. No. 4,496,395 and U.S. Ser. Nos. 414,936 and 544,728, all to Croat and assigned to General Motors Corporation.
Jet casting results in a rapidly solidified alloy ribbon with a very, very fine-grained microstructure which is commensurate with the creation of permanently magnetic properties in the alloys.
In production jet casting operations, it is desirable to provide a jet casting nozzle which is extremely resistant to wear by the flow of molten rare earth-iron alloys. In jet casting nozzles, the size of the orifice increases as alloy flows through it. Before this invention, nozzles made of sintered and machined boron nitride exhibited the longest lives without excessive orifice wear. However, most nozzles did not last more than a few hours before orifice erosion resulted in underquenched ribbons, i.e., alloy flowed through the nozzle too fast and cooled too slowly to result in optimum permanently magnetic properties. Quartz nozzles have also been used to make rapidly solidified RE-Fe alloys but are much shorter-lived than boron nitride nozzles.
This invention relates to novel ceramic compositions based on mixtures of yttrium oxide and aluminum oxide. Nozzles formed by pressing and sintering very fine powders of these compositions have proven to be exceptionally resistant to erosion by molten rare earth alloys.